Background

Field Marshal Lennart Torstenson had unexpectedly marched into Jutland in September 1643 (see Torstenson War). While engaged in operations there, an Imperial army under the command of Count Matthias Gallas ventured north towards Jutland to trap the Swedish army there and destroy it. The Emperor had received requests from Denmark for help, as well as assurance that the Swedish forces were wore down and therefore a fairly easy target.[1] However, since Torstenson thought of Gallas' approaching army of about 15,000 men as a threat to the important Swedish strongholds on the German Baltic coast, he turned his army around and headed south to engage the enemy.[2]

Gallas had his troops build and stay behind abatises and entrenchments south of the river Eider in Holstein in an attempt to trap the Swedish forces on Jutland. This tactic failed, as Torstenson's troops outmanoeuvred the enemy by overthrowing a few Imperial positions and pose a threat both to Gallas' back, and Imperial areas further south. The Imperial army began to move south. During the summer of 1644, Torstenson's forces tried to engage the retreating enemy, and in late September, they had once more caught up with the Imperial army. Gallas responded by ordering his troops to build strong defensive positions and await wished-for reinforcement. The Imperial halting-place, south of Magdeburg, was soon surrounded by the Swedish, who cut off all supplies for Gallas' men. Eventually, the Imperial side ran out of bread and they started to lose people to sickness and starvation. As huge numbers of people and animals died, Gallas saw no other solution but to abandon many of the sick, most of his artillery as well as the baggage, and search protection for his troops in Magdeburg itself. The pattern repeated itself when the Swedish forces managed to enclose the city and cut off the supply. One night, the Imperial cavalry made an attempt to break out.[3]